Polarisation of the light scattered by mercury vapour near the resonance periodicity

Abstract

When light belonging to the visual spectrum is scattered by air, as, for example, in the blue sky, there is no approximation to resonance between any period of vibration contained in the source and a free period of the scattering molecules or atoms. In such cases, there is an approach to complete polarisation of the scattered light, though a closer examination shows that the polarisation is in general incomplete. In argon, however, and probably in the other monatomic gases, there is a very near approach to completeness. In contrast with these cases is the scattering of light by mercury vapour in resonance with the source, when the latter belongs to the ultra-violet mercury line λ 2536, as emitted by a cooled mercury arc. This case was investigated by R. W. Wood. He found that so long as the lamp ran under conditions favourable to producing an extremely narrow line without reversal, there was a copious emission of scattered light. I was anxious, 3 years ago, to trace the transition between this case and the ordinary scattering well away from the resonance periodicity. This was not fully achieved when my attention was turned to other matters, and shortly afterwards my laboratory at the Imperial College was dismantled. I have not since found an opportunity to resume, and it seems desirable to record the results, such as they are, without further delay.